The present invention relates generally to line drivers and more particularly to line drivers having a desired output impedance and required to have a high efficiency.
A line driver is typically used for sending an electronic signal onto a transmission line type medium, such as a copper twisted pair. To avoid unwanted reflections at the far end of the transmission line, the latter is usually matched, i.e., terminated by a far-end impedance element which presents to the transmission line a resistance equal to the characteristic impedance of the line.
In many situations, the transmission medium is bidirectional, meaning that there is a second transmitter at the far end of the transmission line. In order for the transmission line to be properly matched in the reverse direction, the line must xe2x80x9cseexe2x80x9d a near-end impedance equal to its own characteristic impedance. The required near-end impedance, known as the output impedance of the line driver, is usually provided by a reference resistor having a resistance equal to the characteristic impedance of the transmission line. In a voltage-mode configuration, the reference resistor is usually placed in series with the line driver, while in a current-mode configuration, one usually opts for a parallel arrangement.
For example, referring to FIG. 1, there is shown a conventional voltage-mode line driver comprising a voltage source/amplifier combination, shown in dotted outline at 102, connected in series with a reference resistor 106 having a resistance Re. The reference resistor 106 is connected to ground via a transformer 108. The transformer 108 generally has two windings with an equal number of turns, one of which is connected to the reference resistor 106 and the other being used for interfacing with a transmission line 110. The transmission line 110 has a characteristic impedance Zc, which is typically in the range of 50 xcexa9 to 600 xcexa9.
The output resistance of the line driver (denoted Rout) is defined as the ratio of the voltage V0 appearing across the line driver side of the transformer 108 to the current I0 caused by the voltage V0 when the voltage source/amplifier combination 102 is short-circuited. Upon applying the short circuit, it is seen that the voltage V0 appears in its entirety across the reference resistor 106, from which it follows that Rout equals Re. Therefore, in order for the line driver to be matched to the transmission line 110, Rout should equal the characteristic impedance Zc, which means that Re should be set equal to Zc.
Unfortunately, this conventional arrangement results in a wastage of power, as half of the energy output by the voltage source/amplifier combination 102 is lost in the form of heat dissipated in the reference resistor 106. This prevents high-speed modems and other devices that use line drivers from meeting strict power efficiency guidelines. Furthermore, since the amount of circuit card real estate required for a line driver depends on the amount of power that is dissipated, it follows that only a few such drivers can be placed on a circuit card.
A similar scenario occurs in the current-mode dual configuration, now briefly described with reference to FIG. 2. The current-mode line driver comprises a current source/amplifier combination (shown in dotted outline at 202) and placed in parallel with the first winding of a transformer 206. The transformer 206 has a second winding for interfacing with a transmission line 208 having a characteristic impedance Zc. Proper termination at the line driver end is provided by a reference resistor 210 having a resistance Re and also placed in parallel with the transformer 206.
The output resistance Pout of the line driver in FIG. 2 is defined as the voltage V0 appearing across the line driver side of the transformer 206, divided by the current I0 caused by the voltage V0 with the current source/amplifier combination 202 open-circuited. By applying this open circuit condition, it is seen that the voltage V0 appears in its entirety across the reference resistor 210. Therefore, Rout is simply equal to the resistance Re of the reference resistor 210. To achieve proper termination at the line driver end, Rout should be equal to Zc and thus Re is usually set equal to Zc.
Because Re is equal to the characteristic impedance of the transmission line 208, half the energy output by the source/amplifier combination 202 is dissipated in the reference resistor 210. This causes the above-mentioned disadvantages, namely the inability of a conventional line driver to meet power efficiency requirements and the imposition of an undesirably low limit on the number of devices employing line drivers that may be placed on a circuit card.
A known solution is the use of a smaller reference impedance between the output of the line driver and the transmission line and to use a combination of positive and negative feedback around this reference impedance to achieve the desired output impedance. However, one limitation of this method is that the gain of the line driver is low and an additional stage is required at the expense of efficiency and noise performance. Higher gains are achievable but the output impedance is severely affected by tolerances of the components in the positive and negative feedback loops and in the reference impedance.
The above background has shown that there is a need in the industry to provide a line driver which can have the same output resistance as a conventional line driver while reducing the amount of energy or power dissipated in the reference resistor. Furthermore, it would be advantageous to provide a line driver which would also have an independently specifiable gain.
The invention may be summarized according to a first broad aspect as a line driver equipped with an amplifier, a transformer, a reference resistor and a feedback circuit. The amplifier has an input for connection to a voltage source and having an output. The transformer has a first winding and a second winding, the first winding having a first end connected to the output of the amplifier and having a second end and the second winding being connectable to a transmission line having a characteristic impedance. The reference resistor has an end connected to the second end of the first winding at a junction point. The feedback circuit is connected to the input of the amplifier, to the output of the amplifier and to the junction point.
In accordance with this first broad aspect of the invention, the reference resistor has a resistance equal to {fraction (1/K)} times the characteristic impedance of the transmission line and the feedback circuit is arranged to produce a voltage at the output of the amplifier substantially equal to xe2x88x92(Kxe2x88x921) times the voltage at the junction point, for a predetermined value of K.
The resulting output impedance will be equal to K times the reference impedance. At the same time, the voltage across the reference impedance will be reduced by a factor of K, which advantageously reduces the power lost in the reference resistor by a factor of K.
Preferably, the amplifier is an operational amplifier connected in an inverting configuration and the feedback circuit is a resistive network consisting of a first feedback resistor having a first end connected to the output of the amplifier and having a second end connected to the input of the amplifier and also having a second feedback resistor of which a first end connected both to the input of the amplifier and to the second end of the first feedback resistor and of which a second end connected to the junction point. The first feedback resistor preferably has a resistance equal to (Kxe2x88x921) times the resistance of the second feedback resistor.
According to a second broad aspect, the invention may be summarized as a line driver equipped with a transformer, a reference resistor and a feedback circuit. Again, the transformer has a first winding and a second winding, the second winding being connectable to a transmission line having a characteristic impedance. However, in this case, the reference resistor has a first end connected to a first end of the first winding and the feedback circuit has a first port connected both to the first end of the reference resistor and to the first end of the first winding and having a second port connected to the second end of the reference resistor.
The feedback circuit is arranged to draw, through the first port, Kxe2x88x921 times the amount of current flowing through the reference resistor. Hence, the current flowing through the reference resistor will be K times less than the current flowing through the first winding of the transformer, which reduces the losses in the reference resistor while continuing to present a desired output impedance to the transmission line.
Preferably, the feedback circuit is equipped with a first current-to-voltage converter connected to the second port of the feedback circuit, for measuring the current flowing in the reference resistor and a second current-to-voltage converter connected to the first port of the feedback circuit, for measuring the current drawn by the feedback circuit. The feedback also preferably employs a divider circuit and a differential circuit connected to the first and second measuring current-to-voltage converters, for determining the difference between the measured amount of current flowing in the reference resistor and   1      K    -    1  
times the measured amount of current drawn by the feedback circuit. The feedback circuit then generates a compensatory current flowing out the first port, the compensatory current being proportional to the difference so determined.
In accordance with this second broad aspect, the first current-to-voltage converter may be an operational amplifier having an inverting input connected to the second port of the feedback circuit, a non-inverting input connectable to a ground reference and an output and a resistor connected between the input and output of the operational amplifier. The second current-to-voltage converter may be a current transformer connected to the first port of the feedback circuit and having an output.
According to a third broad aspect, the invention may be summarized as a line driver having a transformer, a reference resistor and a feedback circuit. As usual, the transformer has a first winding and a second winding, the second winding being connectable to a transmission line having a characteristic impedance. The feedback circuit has a first port connected to an end of the first winding and also has a second port, a third port and a fourth port. The reference resistor having a first end connected to the second and fourth ports of the feedback circuit and has a second end connected to the third port of the feedback circuit.
In this case, the feedback circuit is arranged to draw, through the fourth port, K times the amount of current flowing through the reference resistor. The current through the reference resistor will be reduced by a factor of K relative to the current flowing through the first winding of the transformer, thereby reducing the energy lost in the reference resistor. Advantageously, the output impedance seen by the transmission line is K times the resistance of the reference resistor.